A plasma display panel (hereinafter referred to simply as a PDP) allows achieving high definition display and a large-size screen, so that television receivers (TV) with a large screen having as great as 100 inches diagonal length can be commercialized by using the PDP. In recent years, use of the PDP in high-definition TV, which needs more than doubled scanning lines than conventional NTSC method, has progressed and the PDP free from lead (Pb) is commercialized in order to contribute to environment protection.
The PDP is basically formed of a front panel and a rear panel. The front panel comprises the following elements:                a glass substrate made of sodium-borosilicate-based float glass;        display electrodes, formed of striped transparent electrodes and bus electrodes, formed on a principal surface of the glass substrate,        a dielectric layer covering the display electrodes and working as a capacitor; and        a protective layer made of magnesium oxide (MgO) and formed on the dielectric layer.The rear panel comprises the following elements:        a glass substrate;        striped address electrodes formed on a principal surface of the glass substrate,        a primary dielectric layer covering the address electrodes;        barrier ribs formed on the primary dielectric layer; and        phosphor layers formed between the respective barrier ribs and emitting light in red, green, and blue respectively.        
The front panel confronts the rear panel such that its surface mounted with the electrodes confronts a surface mounted with the electrodes of the rear panel, and peripheries of both the panels are sealed air-tightly to form a discharge space therebetween, and the discharge space is partitioned by the barrier ribs. The discharge space is filled with discharge gas of Ne and Xe at a pressure ranging from 55 kPa to 80 kPa. The PDP allows displaying a color video through this method: Voltages of video signals are selectively applied to the display electrodes for discharging, thereby producing ultra-violet rays, which excite the respective phosphor layers, so that colors in red, green, and blue are emitted, thereby achieving the display of a color video.
The bus electrodes of the display electrodes employ silver electrodes in order to maintain electrical conductivity, and the dielectric layer employs low-melting glass made of mainly lead oxide. However, in recent years, dielectric layers free from lead for contributing to environment protection have been disclosed in, e.g. patent documents 1, 2, 3, and 4.
However, during the steps of forming the front panel, the silver electrode forming the display electrode diffuses silver ions into the dielectric layer and the glass substrate. The diffused silver ions are subject to the reducing action of alkaline metal ions contained in the dielectric layer and divalent tin ions contained in the glass substrate, thereby forming silver colloid. As a result, the dielectric layer and the glass substrate tend to be yellowed or browned more noisily, and yet, silver oxide having undergone the reducing action generates oxygen which incurs air bubbles in the dielectric layer. The yellowing changes chromaticity, thereby badly degrading picture quality, and what is worse, the air bubbles in the dielectric layer generate failures in insulation of the dielectric layer.
To decrease the yellowing or air bubbles, a double-layer structure is employed to the dielectric layer, namely, the dielectric layer is formed of two layers having different glass compositions. Each one of the two layers is fired during its manufacturing step, so that if air bubbles occur in either one of the layers, the dielectric layer can maintain its electrical withstanding voltage for reducing the failures in insulation.
The respective layers of the dielectric layer have different advantages so that the problems such as the yellowing can be overcome. To be more specific, a lower dielectric layer touching the electrodes of the front panel employs a glass composition which allows reducing the yellowing or air bubbles, and an upper dielectric layer to be formed on the lower dielectric layer employs another glass composition having a higher transmittance.
However, use of multiple dielectric layers differing in materials will complicate the material handling, and eventually increase the cost. On top of that, it invites a misuse of the materials. If the dielectric layer is formed of multiple layers made of only the glass composition employed to the lower dielectric layer, the transmittance of the dielectric layer cannot reach a sufficient level. On the other hand, if the dielectric layer is formed of multiple layers made of only the glass composition employed to the upper dielectric layer, it invites the yellowing and air bubbles.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2003-128430    Patent Document 2: Unexamined Japanese Patent Application Publication No. 2002-053342    Patent Document 3: Unexamined Japanese Patent Application Publication No. 2001-045877    Patent Document 4: Unexamined Japanese Patent Application Publication No. H09-050769